The objective of this research was to design a frequency and polarization agile antenna based on varactors made on ferroelectric thin film. Frequency agility was obtained by changing antenna impedance with ferroelectric based varactor. To obtain polarization agility a system comprising power splitter, tunable filter, phase shifter, and two antennas was designed. BaxSr1-xTiO3 (BST) was chosen for a ferroelectric thin film. Before attempting to implement BST filter, phase shifter, and antenna, interdigital capacitors (IDC) based on various BST compositions and thicknesses have been developed and characterized. Dielectric permittivity and tangent loss of BaxSr1-xTiO3 with x=0.3, 0.4, and 0.5 and thickness between 170 nm and 500 nm were measured at kHz and GHz range. IDC based on ferroelectric thin film substrate were simulated, fabricated and measured. Simulations and measurements were compared, discrepancies validated and possible improvements suggested. Measurement proved that Ba0.3Sr0.7TiO3 with thickness of 240 nm has an optimal balance between tangent loss and tunability and it was chosen to be the substrate for the tunable devices.
Using BST thin films deposited on a Alumina (Al2O3) as a substrate, three antennas (two dipoles and a slot antenna), a phase shifter and a low pass filter have been designed and manufactured. All devices have ferroelectric based varactors integrated in theirs structure in order to obtain frequency agility. Ferroelectric varactors are designed as IDC on BST thin film. Two dipole antennas and a slot antenna were designed with IDC integrated into the feed line or the antenna structure. Coplanar waveguide (CPW) fed dipole antenna has tuning range of 300 MHz and microstrip fed dipole antenna is not tunable at all. Slot antenna has similar tuning characteristics as the CPW fed dipole but because of its broadband characteristic, it satisfies the required resonant frequency range. Antennas radiation diagram, polarization pattern and higher order signals were measured.
Filter was designed as a 5th order low pass Chebyshev filter. By applying bias voltage, its cut-off frequency changes by 500 MHz, which is according to the requirements, but because of divergence between simulations and measurements, it is below the required frequency. Phase shifter is designed as a loaded line phase shifter. It has a 90 ° phase shift
19
at 8 GHz, return loss better than -10 dB and insertion loss better than -2 dB at the whole frequency range. The nonlinear response of the phase shifter and filter was investigated with two-tone intermodulation distortion (IMD) measurement.
|
